The present invention relates to an apparatus for measuring the viscosity of a viscous substance, and more particularly to, a vibration-type viscosity measuring apparatus in which sensitive members are immersed in a viscous substance and subjected to a specific vibration, and the responsive amplitude of the sensitive members is detected to measure the viscosity of the viscous substance.
One type of viscosity measuring apparatus heretofore proposed has a tabular member which is immersed in a viscous substance and vibrated with a predetermined driving power, and the viscous resistance encountered by the tabular member is detected to measure the viscosity of the viscous substance. In such a vibration-type viscosity measuring apparatus, if the vibrating part is not completely fixedly supported, the measured values are inaccurate, thereby resulting in variations in the results of the measurement. Accordingly, the assignee of the present application has previously proposed in Japanese Patent Laid-Open No. 107881/1978 a viscosity measuring apparatus in which two opposing vibrating plates are vibrated at equal frequencies in inverse phase relation to each other for overcoming the above-mentioned disadvantage. By such an apparatus, the reactions of the vibrating plates are constantly cancelled with each other, and it becomes unnecessary to give special consideration to the supporting means for the vibrating plates, so that the results of measurement can be made stable at all times. Although the above-mentioned viscosity measuring apparatus has solved the problem of retaining the vibrating plates, when the driving frequency deviates from the resonance frequency, the responsive amplitude value of the vibrating plates sharply attenuates so as to make it impossible to effect measurement, disadvantageously, which is characteristic of a tuning fork mechanism constituted by two vibrating plates. Also when the relationship between viscosity and responsive amplitude no longer maintains a substantially linear relationship, measurement cannot be performed.
Accordingly, the assignee of the present application has proposed in Japanese Patent Laid-Open No. 135337/1982 a novel viscosity measuring method improved to overcome the above-mentioned disadvantages. More specifically, the method makes it possible to measure even a viscous substance having a small viscosity in such a way that in a viscous substance whose viscosity within a certain range is known, the resonance frequency of the vibrating plates measured when the viscosity is lowest is previously obtained, and the vibrating plates are driven at the resonance frequency. Moreover, when the viscous substance increases in the viscosity so that the relationship between responsive amplitude and viscosity no longer maintains a substantially linear relationship, the driving frequency of the vibrating plates is changed to the resonance frequency of the viscous substance at that time to set a new substantially linear relationship. When the relationship between viscosity and responsive amplitude has deviated from the new substantially linear relationship, the above-mentioned operation is repeated. By so doing, continuous viscosity ranges are determined where viscosity and responsive amplitude have a substantially linear relationship. Then, to which viscosity range a viscous substance whose viscosity is unknown belongs is detected to drive the vibrating plates at a resonance frequency corresponding to the viscosity range and obtain the responsive amplitude thereof, thereby allowing a measurement of viscosity over a wide range. In this measuring method, however, it has been discovered that since each vibrating plate is constituted by a ball, a turbulent flow occurs at the interface thereof thereby making it impossible to obtain a high accuracy. Moreover, it has been found that when a viscous substance is measured in a fluid state, errors are inconveniently produced in the relationship between viscosity and responsive amplitude owing to fluid resistance.